1. Field of the Invention
This invention relates to a sheet processing system and, more particularly, to a system for producing a continuous polymer sheet and winding it on a take-up roll, while providing for smooth and trouble-free take-up roll changes. Specifically, the invention provides a novel construction and arrangement for accumulating the continuously produced polymer sheet while the take-up roll is being changed on the fly.
2. Discussion of the Prior Art
Continuous sheet processing systems are known that provide for take-up roll changes. It is also known to be necessary to provide some form of accumulator between continuous and intermittent in-line processes.
U.S. Pat. No. 3,904,338 to Straumanis discloses a system and method of controlling a machine which continuously produces sheet that is supplied to an intermittently activated in-line processor. The system involves regulation of the sheet extrusion speed along with a vertically oriented and pivotally mounted dancer roller which acts as an accumulator.
Other references disclose various machines for accumulating and temporarily storing a loop of continuous strip material, using a roller loop carriage mounted on a track or cage assembly.
With some continuous sheet processing systems, a plastic sheet is extruded and cooled via a multiple roll stack (sometimes erroneously called a calendar), or a sheet take-off. It is subsequently run over a more or less horizontal stretch of idler rolls for further cooling, thickness measurement, surface treatment, etc. It is then usually edge trimmed to desired width, passed through a set of pull rolls and then wound into rolls on a winder.
These take-up rolls must be changed when they grow to a desired diameter. This creates a problem, especially at increasingly higher speeds because it is difficult to cut the web and transfer it to a new winding shaft in the short time available. This is also dangerous because operators have, at times, been injured in the transfer.
Automatic roll transfer systems exist but are limited to relatively thin plastic sheet because of their limited ability to cut thick plastic sheets. Besides, they are quite expensive. The thickest plastic sheet automatic cut-off system known to be available can cut up to about 0.030xe2x80x3 (0.75 mm) thick sheet. But today it is often needed to wind sheet up to 0.80xe2x80x3 or 0.100xe2x80x3 (2-2.5 mm), or more.
Another frequently used device is a sheet accumulator between the sheet take-off and the winder. This uses a series of (usually vertically arranged) pairs of moveable rolls to accumulate the extruded sheet while the winder is slowed down or even stopped. These accumulators have several disadvantages:
1. They take up excessive valuable space.
2. They are expensive.
3. They require edge trimming after passing through the accumulator because the sheet is never perfectly straight and its edges tend to move laterally as the accumulator stores more or less material.
4. One further great disadvantage arises in building and running compact sheet lines wherein the winder is directly integrated into the downstream end of the sheet take-off. There is no room upstream on a sheet take-off system for the heavy and bulky conventional accumulator.
It is an object of this invention to accommodate cutting and transfer of sheet material of various thicknesses between reels of a sheet take-up system.
It is a further object of this invention to provide a compact integral accumulator, compactly incorporated into the structure of the sheetmaking system, which does not require additional valuable floor or overhead space, and does not require inordinate expenditures for apparatus.
Other objects and advantages of the invention will become readily apparent to those skilled in the art from the drawings and description which follows.
In accordance with this invention there is provided an accumulator that fits the roller conveyor section of the sheet production apparatus. It has unique advantages in that it takes up no room vertically and substantially negligible room horizontally, precedes the usual edge trimmer location, and is very inexpensive to build.
In accordance with this invention there is provided a plastic sheet extrusion system fed by one or more extruders feeding a sheet extrusion die. The die is closely followed by a roll cooling system. The resulting partially cooled sheet is further cooled on a roller conveyor of finite length. This may be horizontal or inclined. (Practically this conveyor is about 10 to 20 ft. long, depending on the product). The conveyor incorporates an accumulator loop (or loops) communicating with its sheet carrying surface. The so arranged accumulator serves to permit roll changes at the winder at a reduced speed or zero speed by appropriate control of the pull roll speed.
The sheet is drawn over the conveyor and through the integral accumulator, which according to this invention is positioned compactly in an upwardly oriented plane just downstream of the conveyor by a pair of driven pull rolls positioned downstream of the conveyor. Passing from the accumulator the sheet is then edge trimmed to the desired width just prior to contacting the pull rolls that convey it to the winder. The sheet is then fed by the pull rolls to a winder which produces rolls of the sheet on a winder reel or takeup roll.
Once the rolls have reached a desired size, the pull rolls slow or stop the feeding of the sheet to the winder to allow transfer of the sheet to a new reel. At this time the accumulator elongates in a generally vertical direction and begins to form the sheet into one or more substantially vertical loops to accommodate the oncoming sheet which is still continuously being produced. Before the accumulator has reached its maximum storage length, ample time has been provided for the sheet to be severed transversely near the reel and transferred to a new reel. Once the transfer has been completed, the pull rolls resume feeding the sheet at a higher speed until the accumulator contracts in a generally vertical direction and reaches its normally empty position, whereupon the pull roll speed is adjusted to approximately that of the incoming sheet.